Ash from untreated wood has a slight liming action and can be used to raise soil pH. Where ash contains larger particles, its incorporation also helps to improve soil structure. Ash produced from young sappy prunings contains a useful proportion of potassium and traces of other nutrients while older wood tends to contain lower concentrations of nutrients. If the ash is collected fresh, it will supply these in a soluble form. The actual nutrient content of ash varies so precise application is difficult. An alternative to soil application is to apply it in thin layers to the compost heap where it blends readily with other materials. Coal ash is best avoided because it has a negligible nutrient content and its fine particle size means it is of little benefit to soil structure.